1. Field of the Invention
The present invention relates to lock systems for securing access to manhole openings.
2. Description of the Prior Art
By way of background, standard manhole covers are designed to be easily removed from manhole openings to allow access to underground facilities such as sewers, electrical and communication equipment vaults, and other infrastructure. This presents a security risk by allowing vandals, terrorists and others to gain unauthorized access to important assets, or to move about undetected via underground passageways.
Various manhole opening locking schemes have been proposed to address such security concerns. One technique is to simply bolt the manhole cover to the underlying manhole frame structure. Although very effective, this method either involves retrofitting existing manhole covers and frames by drilling and tapping bolt holes, or requires that existing covers and frames be replaced with units having preformed bolt holes. The former approach is very labor intensive and both alternatives may be prohibitively expensive if the number of manhole locations is large.
Another manhole security technique involves mounting a lock system to the underside of a manhole cover. A typical lock system includes a pair of retractable pins that extend horizontally to engage the side-wall of the manhole frame or the underside of the ring flange that supports the manhole cover. A long-handled, rotatable security key tool is used to rotate a locking mechanism that actuates the pins into and out of locking engagement. A disadvantage of this technique is its cost. Existing manhole covers must either be replaced with new covers having a pre-mounted lock system, or they must be retrofitted with the lock system. The latter approach requires drilling and tapping bolt holes for securing the lock system, and may also require the formation of an opening in the manhole cover to allow the security key tool to reach the lock mechanism. Alternatively, if the manhole cover has a pre-existing vent hole, this may be used for receiving the security key tool.
Still another manhole security technique involves the use of a lockable pan unit situated below a standard manhole cover. The pan unit is used to block the manhole opening, which means that the manhole cover itself does not require locking and does not have to retrofitted or replaced. The pan unit is secured to the manhole frame by resting it on the same ring flange that supports the manhole cover, and then locking the unit to the manhole frame. A lock system may be used that is similar to those described above for manhole cover-mounted lock systems.
A disadvantage of existing pan-style manhole security devices, as well as some cover-mounted lock systems, lies in their manner of operation. By way of example, U.S. Pat. No. 6,550,294 discloses a pan-style manhole security device wherein a universal joint gear system is provided for actuating a pair of locking pins using a security key tool. Because of the nature of the gear system, many turns of the security key tool are required to move the pins between their locked and unlocked positions, which may be undesirable. U.S. Pat. No. 4,964,755 discloses a manhole cover-mounted lock system wherein the pin-actuating locking mechanism requires only a ninety degree turn of the security key tool to operate a pair of locking pins. However, the locking pins are not self-locking and the security key must be used to return the pins to their locked position once the manhole cover is in place. Moreover, the position of the locking pins in the locked position is fixed by locating pins. Due to dimensional tolerances and differences between manhole frame designs, the locking pins may not firmly engage some manhole frames or may be overly tight in other manhole frames, requiring excessive force to lock the pan. U.S. Pat. No. 5,082,392 overcomes this problem by spring-biasing a pair of locking pins to their locked position. The locking pins affirmatively engage the manhole frame under the force of the biasing springs. A specially configured portion of the security key tool mates with an access aperture in the manhole cover when the locking pins are in their unlocked position. This allows the locking pins to be held in the unlocked position during opening and closing of the manhole opening. However, the security key tool must remain engaged with the manhole cover at all times when the cover is not covering the manhole, which may be inconvenient.
It is to improvements in manhole opening security systems that the present invention is directed. In particular, what is needed is a security device that improves upon previous designs by reducing the effort required to lock and unlock the device, which provides robust locking capability, and which utilizes an uncomplicated design that is easy to manufacture.